Grassland bird diversity and abundance in the presence of native and non-native grazers

Temperate grassland biomes are globally imperiled, and grassland birds are in precipitous decline as a result. The majority of North America's grassland birds breed on rangelands occupied by domestic cattle, but reintroductions of bison to restore evolutionary grazing patterns are increasingly common. Grassland landscapes in western North America have undergone drastic changes since millions of bison occupied this landscape, and the biodiversity effects of bison on modern rangelands remain poorly understood. Here, we test the biodiversity effects of native versus non-native grazers in the context of a highly diverse grassland bird community on the Northern Great Plains of North America. We compared the effects of 2 different grazing treatments common across the region on avian diversity: seasonally grazed and manually rotated domestic cattle, and year-round, free-moving bison. We also test whether estimated abundances of 11 grassland/steppe obligate songbirds differ between these two management regimes. We found roughly equivalent bird diversity, as measured by species richness and Shannon's diversity index, at sites occupied year-round by bison and seasonally by cattle. We also found no significant effect of grazer type on 9 of 11 grassland songbird species. The two exceptions were Grasshopper Sparrow and Vesper Sparrow, which were more abundant in bison pastures. Our results suggest that both native and nonnative grazers can create habitat for a highly diverse assemblage of grassland birds under grazer-specific management regimes.